GB1521252A - Soundness investigation - Google Patents

Abstract

1521252 Investigating structures and materials R J SAVAGE 16 July 1975 [16 July 1974 28 Nov 1974 12 Dec 1974] 31500/74 51660/74 and 53803/74 Heading G1G The soundness or structural integrity of a structure is tested or monitored by transmitting vibrations through it, varying the vibration frequency, sensing the structure's vibration response at several locations of the structure, recording the vibration response frequency spectrum for each location, and either comparing the spectrum with reference data or detecting frequency shifts of the spectral peaks over a period of time. The invention is applicable to detecting fissures, aquifers or other underground anomalies e.g. below the foundations of a building or ahead of a tunnel face being driven; detecting structural changes in off-shore oil and gas platforms or in bridges, railway track, tunnel walls, or building components (beams &c); testing the strength of concrete; and to medical uses such as investigating the soundness of teeth or testing the repair of broken bones. For underground investigations a swept-frequency vibrator is coupled to the earth surface, Fig. 1 (not shown), to a vertical borehole wall, Fig. 4 (not shown), or to a tunnel face, Fig. 3A (not shown) and vibration sensors are located respectively at the earth surface, in further vertical boreholes, or in holes bored ahead of the tunnel face. Voids, &c, in the ground are detected by the resonances produced in the vibration response sensed and are localized by triangulation, using rotatable sensors. Each sensor comprises a triaxial accelerometer. Acoustic coupling of the vibrator to the ground is afforded by a saucer-shaped member held against the ground by vacuum. Fig. 5 shows a vibrator for generating shear waves in a borehole. It is anchored by a gas-filled device 26 and comprises an electrically insulated tube 20 filled with a gel in which are suspended magnetizable metallic particles 25. Application of an A.C. signal across end electrodes 23 is said to produce oscillation of the vibrator in the direction of the borehole axis. For monitoring the structural integrity of an off-shore platform at least one vibrator and several sensors are fixed above the waterline to various structural members and the vibration response is recorded digitally at intervals (of days or longer). Additionally, strain gauges are fixed to structural members below the waterline. Fig. 13 shows such a gauge, of the vibrating-wire type, in which the wire 45 is within a tube 44 fixed at least at its ends to a member. Electromagnets 46 vibrate the wire transversely in an upper harmonic mode and are used detect the vibration frequency. Alternatively a longitudinal-wave mode gauge can be used, Fig. 14 (not shown), or the gauge may comprise a water-filled tube with means for timing the two-way propagation of a shock-wave along the water column, Fig. 15 (not shown).